I've heard the following two sentences and I'd like to understand the differences between the two, if any, and when it is appropriate to use one vs the other:
おいしゃさんとはなしています
おいしゃさんにはなしています
It is my understanding that these both translate to "I am speaking to the doctor", but what is the difference between と and に here?
おいしゃさんとはなしています is "I am talking with my doctor", whereas おいしゃさんにはなしています is "I am talking to my doctor".
The first one with と is natural in most cases, because when you go to a doctor, normally both you and your doctor will talk to each other.
～にはなす can be naturally used when you're talking to something that do not reply to you:
彼【かれ】は人形【にんぎょう】に話【はな】している。
He is talking to a doll.
...or what is important is whom you are talking to:
私【わたし】はあなたにではなく、お医者【いしゃ】さんに話【はな】しています。
I am talking not to you, but to my doctor!
It is the same as the difference between talking 'with' someone as opposed to talking 'to' them. おいしゃさんにはなしています can mean that some information is being conveyed to the doctor (basically 'telling' them). と does not imply such a nuance.
お医者{いしゃ｝さんに言｛い｝う would be simply conveying that information, but お医者｛いしゃ｝さんに話｛はな｝す implies discussion relevant to the information might also be taking place.
彼はあなたに話していますよ、先生。He's talking to you, doctor.
